The present invention relates to a semiconductor design technology; more particularly, to a semiconductor memory device capable of measuring a temperature without the influence of noise.
Generally, in semiconductor memory devices, a cell includes a transistor, as a switch, and a capacitor storing electric charges (data). Since the data storage is carried out by the charges which are accumulated in the capacitor, power consumption is not caused basically. However, the initially stored charges can be extinguished because there is a leakage current which is caused by the PN junction of MOS transistor. Then, this leakage current causes a loss of the stored data. To prevent this problem, a recharging operation should be carried out before the data loss and this recharging operation is achieved by reading out the data from the memory cell and then recharging the memory cell again based on the read-out data.
The stored data are maintained only when this recharging operation is periodically repeated. The recharging process of the electric charges in the memory cells is called refresh operation and the refresh control is achieved by a DRAM controller. In DRAMs, power consumption is created due to the need of the refresh operation. It is very important to reduce the power consumption in battery operated system, such as portable electronic device which demands the low power consumption and it is a critical issue these days.
One of various attempts to reduce the power consumption required in the refresh is to diversify the refresh cycle according to a temperature. The data retention time in the DRAM is lengthened as a temperature is decreased. Therefore, if the temperature field is divided into the different domain frames and the frequency of the refresh clock is relatively lowered in the low temperature field, the power consumption is decreased. Accordingly, a device is needed to sense the temperature in the DRAM and to output the information on the sensed temperature.
Moreover, with the increase of the integration and working speed in the semiconductor memory devices, heat is generated more and more in the semiconductor memory device itself. This generated heat increases the inner temperature in the semiconductor memory device and the inner temperature disturbs the normal operation. The inner temperature can cause the inferiority of the semiconductor memory device and can function as a reason why the semiconductor memory device itself is damaged. Therefore, the temperature of the semiconductor memory device should be accurately sensed. Accordingly, a device is needed to exactly sense the temperature in the DRAM and to output the information on the sensed temperature.
FIG. 1 is a block diagram of a conventional temperature sensing device in a semiconductor memory device.
Referring to FIG. 1, the conventional temperature sensing device includes a temperature sensing unit 10 to sense a temperature in response to a driving signal ODTS_EN, an ADC (Analog to Digital Converter) 20 to convert an analog signal from the temperature sensing unit 10 into a digital signal, and a register 30 to store a digitalized temperature value form the ADC 20.
The operation of the conventional temperature sensing device will now be described.
First, when the driving signal ODTS_EN is activated, the temperature sensing unit 10 senses the current temperature in response to the activated driving signal ODTS_EN and outputs an analog temperature value. Subsequently, the ADC 20 converts the analog temperature value into a digital signal. The register 30 stores the output from the ADC 20 and outputs it as a temperature value TM_VL.
However, the driving of the temperature sensing device as described above is unable to reflect the current temperature. It drops down the reliability of data or it induces the excessive power consumption. This is because the driving of the temperature sensing device can be performed together with the driving of other devices within the semiconductor memory device. As a result, an error can occur in the temperature value due to the noise which is generated by the driving of other circuits.
A voltage unstable situation, such as a voltage drop, a ringing phenomenon or oscillation, is generated by the current and voltage consumption according to the operations of the devices. When the sensed temperature is converted into the digital value, an error is caused by the fluctuation which is generated by the unstable voltage or current.
As to the refresh in which a cycle is determined by the temperature value of a device, the refresh is unable to be performed in an appropriate cycle due to the erroneous temperature value. This inappropriate refresh can cause a loss of the stored data and reduce the reliability of the memory device. Furthermore, the excessive refresh induces the unnecessary power consumption.
A semiconductor memory device is generally manufactured with a RAS timing having a predetermined value according to an initial design of configuration. However, after the semiconductor memory device is manufactured, the refresh operation may be performed differently, compared with the initial design. At this time, the predetermined RAS timing can be too long or short for the semiconductor memory device to perform the refresh operation. If the RAS timing is too long, a refresh operation time also increases and too much current is consumed. On the other hand, if the RAS timing is too short, the refresh operation is not performed sufficiently and the stored data are lost.